The objective of the study is to investigate the efficacy and safety of linagliptin 2.5 mg twice daily compared to 5 mg once daily compared to placebo given orally for 12 weeks as add-on therapy to metformin in patients with type 2 diabetes mellitus with insufficient glycaemic control. It is planned to show non-inferiority of linagliptin 2.5 mg twice daily compared to 5 mg once daily and each treatment's superiority over placebo.